https://leetcode-cn.com/problems/4sum/
双指针解
class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> result;
        sort(nums.begin(),nums.end());
        for(int i = 0 ; i < nums.size() ; i++)
        {
            if(i > 0 && nums[i] == nums[i-1])
                continue;
            
            for(int j = i + 1 ; j < nums.size() ; j++)
            {
                if( j > i + 1 && nums[j] == nums[j - 1])
                    continue;
                int left = j + 1;
                int right = nums.size() - 1;
                while(right > left)
                {
                    long res = (long)nums[i] + (long)nums[j] + (long)nums[left] + (long)nums[right];
                    if(res > target)
                        right--;
                    else if(res < target)
                        left++;
                    else
                    {
                        result.push_back(vector<int>{nums[i] , nums[j] , nums[left] , nums[right]});
                        while(right > left && nums[right] == nums[right -1]) right--;
                        while(right > left && nums[left] == nums[left +1]) left++;
                        left++;
                        right--;
                    }
                }
            }
        }
        return result;
    }
};